The present invention relates to a driving technique for an electro-optical display which utilizes variations in the light absorption properties upon application of properly controlled potentials.
In the art of electro-optical displays, transition metal oxide materials of for example tungsten oxide (WO.sub.3), molybdenum oxide (M.sub.o O.sub.3) or titanium oxide (T.sub.i O.sub.2), organic materials of for example viologen derivatives (4,4' bipyridinium derivatives), etc. are well known as electrochromic materials employed for display purposes. Although in the following description the present invention will be discussed and illustrated with respect to the bipyridinium derivatives, it is obvious that the principle of the present invention is equally applicable to other electrochromic materials. Other materials employed instead of the 4,4' bipyridinium derivatives are 2,2' bipyridinium derivatives; 2,4' bipyridinium derivatives; 2-(2',4' dinitrobenzyl) pyridine; N,N'-dimethyl 9,9'-biacridinium dinitrate: etc. See, for example, British Pat. No. 1,302,000 of PHILIPS ELECTRONIC AND ASSOCIATED INDUSTRIES LTD. and entitled IMAGE DISPLAY APPARATUS.
A scheme of such an electrochromic display cell is shown in FIG. 1. As shown herein, on a substrate 1 of glass, ceramics or plastics there is first deposited an electrically conductive coating 2 of inert metal, indium oxide (In.sub.2 O.sub.3), tin oxide (SnO.sub.2), etc. A layer of insulator 3 made of oxides such as silicon monoxide (SiO), silicon dioxide (SiO.sub.2) and aluminium oxide (Al.sub.2 O.sub.3), or fluorides such as magnesium fluoride (MgF.sub.2) or polymer material such as resist ink and photoreist, is then deposited on only a non-display region, that is, a region not serving for display purposes. The conductive coating 2 in combination with the insulator layer 3 having a desired pattern defines the corresponding display pattern. The conductive coating 2 may be termed display electrodes. A layer of the electrochromic material 4 (for example, the bipyridinium derivative as listed above) is further deposited thereon. In case where the electrochromic materials are fluid, a second substrate 8 of glass, ceramics or plastics is additionally provided such that the electrochromic material 4 may be injected via a spacer 5 into a cavity between the two substrates 1 and 8. The spacer 5 may be made of a glass rod or plastic rod. A counter electrode 6 and, if necessary, a reference electrode 7 are deposited on the second substrate 8. The counter electrode 6 may be made of inert metal, tungsten oxide (WO.sub.3) or resin-like carbon, while the reference electrode 7 may be made of indium oxide (In.sub.2 O.sub.3). The resulting electrochromic display cell is analogous in construction to the conventional liquid crystal display cells with exceptions that while the former manifests color reactions at the interface between the electrochromic material containing layer (solid, gel or fluid) and the display electrodes, in case of the latter changes in the optical characteristics occur only on the portion which is sandwiched between the two electrodes and a voltage is applied thereto. There is observed another difference that when a constant potential driving technique effective to hold the potentials of the display electrodes constant is employed, the electrochromic display needs the provision of the reference electrode in addition to the counter electrode.
These electrochromic displays are of greater advantage than the prior art displays because of its lower power and memory capacity. The memory capacity means that the display is capable of holding its display state after being electrically free from a driving circuit and thus the display requires no power dissipation at this time. These advantageous characteristics are found very useful within a wide range of fields where the frequency of occurrence of changes in the display state is relatively small and the requirement for lower power dissipation is rigid.
When the bipyridinium derivative for example is employed, the bipyridinium derivative is reduced about the display electrodes or cathodes so that insoluble colored species are deposited on the surfaces of the electrodes to effect the color reactions. Upon the reversal of the polarities of these electrodes, the derivative is oxidized to restore to its original state. Accordingly, the memory characteristics will be further enhanced if the oxidative species are removed during the fabrication of the display cells. However, difficulties are experienced to erase the colored state. Repeated write-erase cycle permits the colored species to be accumulated on the electrodes to thereby shorten the operating life of the display cells. To overcome the above discussed difficulties, a redox pair (for example, Fe.sup.2+ and Fe.sup.3+) is added which has an appropriate redox potential (preferably, 0.1-1 V higher than the redox potential or the oxidation-reduction potential of bipyridinium derivatives). Another example of the redox pair is tetrachlorohydroquinone/tetrachloroquinone. In these cases, erasure is readily effected and overvoltage at the counter electrode is minimized. Also, the driving voltage is reduced. Notwithstanding these improvements, difficulties still remain in that the memory characteristics are considerably declined.
Accordingly, it is an object of the present invention to provide an improved driving manner for electrochromic display cells which is capable of overcoming the shortcomings experienced in the prior art. In accordance with the concept of the present invention, two step potentials are applied to the electrochromic display cells during a single write-erase cycle. In other words, the present invention is characterized by the driving technique for the electrochromic display wherein a redox pair having an appropriate redox potential is added to bipyridinium derivative, for example, diheptyl viologen dipromide (N,N'-diheptyl 4,4' bipyridinium dibromide), the driving technique being capable of enhancing the memory capacity of the electrochromic display.